1. Field of the Invention
The present invention relates to a thin film forming apparatus which can simultaneously realize both strong reactivity that is the merit of CVD (Chemical Vapor Deposition), and the film formation under a high vacuum that is the merit of PVD (Physical Vapor Deposition).
2. Description of the Related Art
There are known thin film forming apparatus utilizing CVD and PVD. The apparatus utilizing CVD have the merit of providing strong reactivity, and the apparatus utilizing PVD have the merit of enabling the formation of a dense and strong thin film under a high vacuum.
Various types of thin film forming apparatus utilizing CVD and PVD have been proposed in the past. Any of those apparatus, however, has problems that an adhesion between the thin film formed on a substrate and the substrate is poor, and that it is difficult to form a uniform thin film, when the thin film is formed over a large-area substrate.
To solve those problems, there are further known, as developed forms of the above ones, thin film forming apparatus utilizing the so-called ion plating and DC ion plating methods (see Japanese Patent Publication No. 52-29971 (1977) and 52-29091 (1977), for example). In the former apparatus using the ion plating method, a high-frequency electromagnetic field is produced between a source of evaporation and an object undergoing vapor deposition to ionize the substance which is evaporated in an active gas or an inert gas, so that the evaporated substance (or substance vapor) adheres onto the object undergoing vapor deposition in vacuum to form a thin film thereon. In the latter apparatus using the DC ion plating method, a DC voltage is additionally applied between the source of evaporation and the object undergoing vapor deposition.
As a further developed type, there is proposed a thin film forming apparatus in which a substrate for depositing a thin film thereon is held on a counter electrode in opposed relation to a source of evaporation, a grid is disposed between the counter electrode and the source of evaporation, and a filament for thermionic emission is further disposed between the grid and the source of evaporation, with the grid being impressed with a positive potential relative to the filament, thereby to form a thin film (see Japanese Patent Laid-Open (Kokai) No. 59-89763 (1984)). According to this structure, the substance evaporated from the source of evaporation is ionized by the thermions emitted from the filament. The ionized substance which passes through the grid is accelerated by the action of an electric field which is directed from the grid to the counter electrode and impinges upon the substrate for deposition. A thin film having a good adhesion is thus formed on the substrate.
The thin film forming apparatus of the above-mentioned developed type, however, has several problems. The adhesion between the thin film formed and the substrate for deposition has been improved to some degree, but not yet strong enough. It is also difficult to form a thin film on a substrate having a low heat resistance.
Another problem is that in the case of forming various multi-composition thin films of magnetic alloys, oxide superconductors, semiconductors and the like, particularly ITO (Indium Tin Oxide) and other films of which characteristics are considerably changed by doping of a trace element, it is difficult to effectively take the trace element into the thin films. Thus, thin films having desired composition ratios are hard to form.
Still another problem is that in the case of forming a thin film over a large-are substrate, although the electric field produced by the grid acts to make the distribution of film thickness on the substrate surface more uniform than can be in the case of usual vacuum vapor deposition without applying such an electric field, the distribution of film thickness is largely affected by the positional relationship between the source of evaporation and the counter electrode. This gives difficulties in forming a thin film having a uniform thickness.
Furthermore, in the thin film forming apparatus of the above-mentioned developed type, ionization of the evaporated substance is accelerated by the thermions emitted from the filament, and most of those thermions and secondary ions generated upon the ionization are finally absorbed by the grid. Thus, the electron density that largely effects the ionization of the evaporated substance tends to become uniform, if the grid has a uniform shape. On the contrary, this leads to a problem that the spacing between the source of evaporation and the subtrate for deposition, etc. is liable to directly affect the distribution characteristics of film thickness.
Moreover, the evaporated substance is radially emanated from the source of evaporation, but scattered by the collisions with gas molecules and ions. Among the evaporated substance thus scattered, those vapor particles ionized by the collision with the thermions and the like are caused to impinge upon the substrate perpendicularly under the action of the electric field directed from the grid toward the substrate, while many of non-ionized vapor particles are likely to impinge upon the substrate obliquely. This leads to a problem of making it rather difficult to form a uniaxially oriented thin film due to different incident directions and angles of those non-ionized particles with respect to the substrate.